The p110δ Isoform of Phosphatidylinositol 3-Kinase Controls Susceptibility to Leishmania major by Regulating Expansion and Tissue Homing of Regulatory T Cells¹

Leishmaniasis, one of the six major tropical diseases identified by the World Health Organization for intense further study, is a chronic protozoan disease that is endemic in 88 countries and affects more than 12 million people. A key event in the life cycle of this protozoan is infection of macrophages and dendritic cells, which need to become activated to kill these intracellular parasites. The activation of infected macrophages is regulated by the availability of IFN-γ produced by activated T cells. Thus, a strong T cell response is thought to be necessary for resistance to Leishmania major in humans.

L. major-infected mice mimic the human cutaneous disease, with healing and nonhealing disease in specific mouse strains, dependent on the type of CD4⁺ Th cell subset that is generated (1, 2). Traditionally (and as in humans), healing has been associated with the development of a strong Th1 response, leading to the production of IFN-γ, which activates macrophages, ultimately resulting in killing of parasite in these cells (1, 3). Nonhealing disease has been associated with the development of Th2 cells that produce IL-4, a cytokine that deactivates macrophages, making them unable to kill the intracellular parasites (1, 3). IL-10 is another cytokine that has been shown to regulate disease outcome. IL-10-deficient mice are highly resistant to L. major (4), and transgenic overexpression of IL-10 renders resistant mice susceptible (5). The cellular source of IL-10 has been shown to include natural CD4⁺CD25⁺ regulatory T cells (Tregs)³ (6, 7), conventional CD4⁺ T cells (8), and in some cases, macrophages (9). Accordingly, C57BL/6 mice, in which Th1 immune responses predominate, clear Leishmania effectively whereas BALB/c mice, which preferentially mount Th2 immune responses, are susceptible and fail to control Leishmania infections.

The class IA PI3Ks are a family of p85/p110 heterodimeric lipid kinases that control multiple cellular processes, including cell differentiation, growth, proliferation, migration, metabolism, and survival (10). There is accumulating evidence for an important role of PI3Ks in the immune response (10, 11, 12). Mammals have three catalytic subunits of class IA PI3Ks (p110α, p110β, and p110δ) (13), with the p110δ isoform being highly enriched in leukocytes (14). L. major promastigotes can activate PI3K/AKT signaling in infected host macrophages and confer resistance to apoptosis, thereby giving ample time for parasites to complete their replication cycle (15). In contrast, macrophage-specific inhibition of PTEN (phosphatase and tensin homolog deleted on chromosome 10), a phosphatase that negatively regulates PI3K pathway (16, 17), abrogates efficient killing of parasites by infected macrophages (18). Thus, the PI3K pathway has been shown to either promote or impede parasite growth in macrophages in different experimental model systems. PI3Ks have also been shown to negatively regulate TLR signaling in dendritic cells (DCs). Thus, p85^−/− mice show enhanced production of IL-12 leading to increased Th1 responses and resistance to L. major infection (19). However, p85a^−/− mice show no T cell defects, and hence the role for T cell-intrinsic PI3K activity was not addressed in these studies.

In contrast, inactivation of p110δ protein by germline knock-in of an inactivating mutation (herein referred to as p110δ^D910A) results in impaired T cell proliferation and cytokine (IL-2, IL-4, and IFN-γ) production in response to Ag stimulation in the presence of LPS (20, 21). However, in response to challenge by Ag adsorbed to alum, IFN-γ responses were either unaffected or slightly enhanced, probably due to impaired IL-10 production (22). Indeed, p110δ^D910A mice have fewer peripheral CD4⁺CD25⁺Foxp3⁺ T cells, which secrete less IL-10 and fail to suppress colitis (11, 20). Given that p110δ inhibition can variably block both inflammatory cytokine signaling and T cell regulation, it was difficult to predict how p110δ^D910A mice might respond to challenge with a parasite.

To address this, we investigated the outcome of infection of p110δ^D910A mice with the intracellular pathogen L. major. Our results reveal that even in the context of diminished Th1 responses, p110δ deficiency restricts the growth of L. major in vivo by preventing expansion, tissue homing, and effector activities of natural Tregs.

Female C57BL/6 mice (B6, wild type (WT)), BALB/c mice, and CB-17/lcr-Prkdc^scid/Crl (BALB/c SCID) were purchased from Charles River Laboratories. B6 mice that express an inactive form of p110δ isoform of PI3K (named p110δ^D910A) were generated by introducing a germline point mutation into the p110δ gene as previously described (20). BALB/c p110δ^D910A mice were generated by backcrossing B6/129 p110δ^D910A mice onto the BALB/c background for >12 generations. All mice were maintained at the University of Manitoba Animal Care facility under specific pathogen-free conditions and used according to guidelines stipulated by the Canadian Council for Animal Care.

L. major parasites (MHOM/80/Fredlin) were grown in Grace’s insect culture medium (Invitrogen) supplemented with 10% heat-inactivated FBS (HyClone), 2 mM glutamine, 100 U/ml penicillin, and 100 μg/ml streptomycin. For infection, mice were injected with 5 × 10⁶ (C57BL/6) or 2 × 10⁶ (BALB/c) stationary phase promastigotes in 50 μl of PBS suspension into the right hind footpad. Lesion sizes were monitored weekly by measuring footpad swellings with calipers. Parasite burden in the infected footpads was determined by limiting dilution assay.

At sacrifice, infected footpads were homogenized in 5 ml of PBS supplemented with 10% FBS using tissue grinders and centrifuged at 300 rpm for 5 min to removed large tissue debris. The supernatant was then layered on top of 10 ml Ficoll and centrifuged at 2000 rpm for 30 min at 20°C. The layer of low-density cells at the interface was harvested and stained for CD3, CD4, CD8, and CD25 expression for flow cytometry analyses (see below).

Infected mice were sacrificed at different time points after infection and the draining popliteal lymph nodes (dLNs) were collected and made into single-cell suspensions. Cells were resuspended at 4 × 10⁶/ml in DMEM supplemented with 10% heat-inactivated FBS, 2 mM glutamine, 100 U/ml penicillin, 100 μg/ml streptomycin, 25 mM HEPES, and 5 × 10⁻⁵ M 2-ME (complete medium), plated at 1 ml per well in 24-well tissue culture plates, and stimulated with soluble Leishmania Ag (SLA; 30 μg/ml) or soluble anti-CD3/CD28 mAb (1 μg/ml; BioLegend).

To measure proliferation, cells were labeled with CFSE (Molecular Probes) according to the manufacturer’s suggested protocols. The labeled cells were resuspended at 10⁶/ml, plated onto 96-well round-bottom plates, and stimulated with SLA or soluble anti-CD3 and anti-CD28. After 5 days, proliferation was analyzed by flow cytometry.

Three days after culture, the supernatant fluids were collected and assayed for cytokines (IFN-γ, TNF-α, IL-4, and IL-10) by ELISA using paired Abs (BD Biosciences) according to the manufacturer’s suggested protocols. The cells were used for intracellular cytokine staining according to standard protocols. Briefly, cells were stimulated with 50 ng/ml PMA, 500 ng/ml ionomycin, and 10 μg/ml brefeldin A (all from Sigma-Aldrich) for 4 h before surface staining. Fixed and surface-stained cells were permeabilized with 0.1% saponin (Sigma-Aldrich) in staining buffer and then stained with specific fluorochrome-conjugated mAbs against Foxp3, IFN-γ, IL-4, and IL-10 (BioLegend). Samples were acquired on a FACSCalibur and analyzed using CellQuest Pro (BD Biosciences).

Bone marrow cells were isolated from the femur and tibia of both WT and P110δ^D910A mice. The cells were differentiated into macrophages using complete medium supplemented with 30% L929 cell culture supernatant. BMDCs were differentiated in petri dishes in the presence of rGM-CSF (20 ng/ml; Peprotec). Immature DCs were harvested on day 7 and assessed for the expression of CD11c, CD40, CD80, CD86, and MHC class II by flow cytometry. The BMDMs and BMDCs were infected at a cell-to-parasite ratio of 1:5. After 5 h of infection, free parasites were washed away and infected cells were further stimulated with different concentrations of IFN-γ or LPS for 24 h. The ability of the cells to kill parasites was determined by counting Giemsa-stained cytospin preparations under light microscope at ×100 (oil) objective.

At sacrifice, serum was obtained from infected mice and used to determine the levels of anti-Leishmania-specific Ab titers by ELISA. At 1, 2, and 4 wk postinfection, WT or p110δ^D910A mice were injected i.p. with pooled serum (500 μl/mouse/injection) from naive or 6-wk-old L. major-infected mice.

WT and p110δ^D910A mice were injected with purified anti-IFN-γ mAb (XMG1.2, 2 mg/mouse) 1 day before infection with 5 × 10⁶ L. major, followed by weekly treatment with mAb (1 mg/mouse) for 6 wk. Lesion sizes were monitored weekly and, at sacrifice, parasite burden in the infected footpads was determined by limiting dilution.

Thy1.2⁺, CD4⁺CD25⁺, and CD4⁺CD25⁻ T cells were purified from pooled spleen and lymph node cells from naive or infected WT and p110δ^D910A mice by negative selection (Thy1.2⁺) or a combination of positive and negative selection using autoMACS column and Abs according to the manufacturer’s suggested protocols (Miltenyi Biotec). Purified cells were >98% pure as assessed by flow cytometry. One million CD4⁺CD25⁺ and 2 million CD4⁺CD25⁻ cells from naive or infected WT (BALB/c) mice were adoptively transferred by i.v. into naive p110δ^D910A mice that were subsequently infected with L. major the next day. In some experiments, SCID BALB/c mice were reconstituted with 10 million Thy1.2⁺ cells from WT or p110δ^D910A BALB/c mice and infected the next day with L. major.

Splenic T cells from WT and p110δ^D910A mice were purified using a T cell enrichment kit according to the manufacturer’s suggested protocols (StemCell Technologies). Purified T cells were stimulated in vitro for 3 days with plate-bound anti-CD3 mAb (BD Biosciences) in the presence of soluble anti-CD28 and rIL-2 (50 U/ml; BD Biosciences). Total cellular RNA was extracted from stimulated T cells using TRIzol reagent (Invitrogen), and cDNA was synthesized from 1 μg of RNA using SuperScript II reverse transcriptase (Invitrogen). RT-PCR was performed using a LightCycler System (Roche Diagnostics). The primers for Blimp-1 were selected according to a previously published report (23).

A two-tailed Student’s t test was used to compare means of lesion sizes, parasite burden, and cytokine production from different groups of mice. Significance was considered if p ≤ 0.05.

p110δ^D910A mice have impaired T cell responses (proliferation and cytokine production) following polyclonal and/or model Ag stimulation (21). Surprisingly, L. major-infected p110δ^D910A mice developed smaller cutaneous lesions that resolved faster than in the usually resistant C57BL/6 (WT) mice (Fig. 1,A). The accelerated lesion resolution was accompanied by faster parasite control at infection site (Fig. 1,B) and its dLNs (data not shown). By 2 wk postinfection, p110δ^D910A mice harbored fewer parasites than did WT mice, and this reduction was maintained for several weeks (Fig. 1 B, wk 5 postinfection is shown). At 10 wk postinfection, p110δ^D910A and WT mice completely resolved their footpad lesions and harbored similarly low but detectable numbers of parasites at the infection site, consistent with the characteristic persistence of parasites in healed mice (24, 25).

The enhanced resistance in p110δ^D910A mice prompted us to assess their T cell responses following Leishmania infection. Infected p110δ^D910A mice had fewer leukocytes than did WT mice in the infected footpads, dLNs, and spleens (Fig. 1,C and data not shown). Interestingly, while there was no different in the percentage of CD11b⁺ cells in the footpads of infected WT and p110δ^D910A mice (Fig. 1,D, left panel), there was a significant difference in the quality (subset) of T cell infiltration into this tissue (Fig. 1,D, right panel). Most T cells in the infected footpads of WT mice at 5 wk postinfection were CD4⁺ (85%), while only ∼40% of cells recovered from footpads of infected p110δ^D910A mice were CD4⁺, with the majority being CD8⁺ cells (Fig. 1,D, right panel). However, following lesion resolution (10 wk), the percentage of CD4⁺ T cells in the footpad became similar (34% vs 39% in WT and p110δ^D910A mice, respectively). These data suggest that CD4⁺ cells from infected p110δ^D910A mice may be defective in clonal expansion in the dLNs or that their recruitment to inflamed sites is impaired. Consistent with the observed lower number of cells in the dLNs and tissues, cells isolated from infected p110δ^D910A mice were significantly impaired in recall proliferative response to soluble leishmanial Ag in vitro (SLA; Fig. 1,E) and polyclonal (anti-CD3⁺ anti-CD28 mAb) stimulation (data not shown). Interestingly, cells from p110δ^D910A mice were significantly impaired in their ability to make cytokines, particularly IFN-γ, IL-10, and TNF (Fig. 1, F and G, and data not shown). This impaired response was observed throughout the course of the infection (Fig. 1 H). Thus, despite showing a blunted IFN-γ response, p110δ^D910A mice exhibit enhanced resistance to L. major.

Next, we investigated if the enhanced resistance in the absence of p110δ signaling is mouse strain specific. BALB/c mice are highly susceptible to L. major infection because they develop strong Leishmania-specific T cell proliferation associated with high IL-4 and IL-10 production (26). In contrast to WT BALB/c mice, which developed progressive and nonhealing ulcerative lesions, L. major-infected p110δ^D910A BALB/c mice (12th generation backcross) developed minimal lesions (and in some cases no lesion at all) upon infection with L. major (Fig. 2, A–C; note that this experiment was terminated after 5 wk when the footpads of WT mice began to ulcerate). Similar to infected p110δ^D910A C57BL/6 mice, T cells from the dLNs and spleens of infected p110δ^D910A BALB/c mice proliferated less (data not shown) and fewer cells produced cytokines (IL-4, IL-10, IFN-γ and TNF) after stimulation with SLA than did those from WT mice (Fig. 2, D–G). We conclude that the loss of p110δ activity is sufficient to convert the normally susceptible BALB/c mice to become resistant to Leishmania infection and that the enhanced resistance to L. major following inactivation of p110δ is independent of genetic background.

Because the route of L. major infection influences the nature of the immune response (27, 28), we investigated whether the concomitant suppression of immune response and enhanced resistance of p110δ^D910A mice is dependent on the route of infection. As shown in Fig. 3, A–D, the pattern of resistance and immune response in p110δ^D910A mice following intradermal (ear) infection was similar to those seen following s.c. (footpad) infection, suggesting that the enhanced resistance of p110δ^D910A mice to L. major is independent of route of infection.

p110δ^D910A mice have reduced numbers of peripheral B cells as well as impaired B cell signaling leading to a reduction in serum Ab levels and total numbers of circulating plasma cells (20, 29, 30). Serum levels of anti-Leishmania IgG Abs became detectable at 2 wk postinfection infection, peaked at 5 wk, and plateaued thereafter in both WT and p110δ^D910A mice. Consistent with the reported impaired B cell signaling, the total IgG as well as parasite-specific IgG1 and IgG2a levels in the sera of infected p110δ^D910A mice were significantly lower than in WT controls (Fig. 3 E).

High levels of anti-Leishmania Abs may enhance disease by facilitating uptake of amastigotes by macrophages via the FcγRII (31, 32), although this concept has recently been challenged (33). Nonetheless, to determine whether the low Ab response (see Fig. 3,E) contributed to the enhanced resistance of p110δ^D910A mice to L. major, we injected them with normal serum (from uninfected) or serum obtained from 6-wk L. major-infected WT mice (a time when anti-Leishmania IgG levels were highest in infected WT mice) weekly from −1 to +3 wk postinfection and compared their lesion development after infection. p110δ^D910A mice given either normal or infected sera were still highly resistant to L. major despite mounting impaired cytokine responses (Fig. 3, F–H), suggesting that impaired B cell responses in p110δ^D910A mice are not responsible for their enhanced resistance to L. major.

Next, we determined if the enhanced resistance of p110δ^D910A mice was related to hyperactivity of their macrophages and/or DCs. BMDMs and BMDCs from p110δ^D910A mice produced comparable or higher levels of IL-12 in response to IFN-γ (Fig. 4,A) or LPS (Fig. 4, B and C). Similarly, ex vivo splenic DCs from p110δ^D910A mice also produced more IL-12p40 spontaneously or upon LPS and anti-CD40 mAb stimulation than did those from WT mice (Fig. 4,D). However, both WT and p110δ^D910A BMDMs and BMDCs were equally permissive to L. major infection, allowing parasite proliferation in vitro at comparable levels (Fig. 4, E–G), and there was no significant difference in the ability of p110δ^D910A and WT cells to kill L. major following stimulation with IFN-γ (Fig. 4,F) or LPS (Fig. 4,G). Consistent with this, uninfected and L. major-infected macrophages from WT and p110δ^D910A mice produced similar levels of NO and reactive oxygen radicals following IFN-γ and LPS stimulation (Fig. 4,H and data not shown). Taken together, these results show that the enhanced resistance of p110δ^D910A mice to L. major infection is not due to enhanced macrophage responsiveness. The reduced T cell IFN-γ response (Figs. 1 and 2) also argues against a significant role for the enhanced secretion of IL-12 by the DCs and macrophages in this model system.

Resistance to L. major is primarily dependent on IFN-γ produced by CD4⁺ and CD8⁺ T cells. Given that IFN-γ production by cells from L. major-infected p110δ^D910A mice is significantly impaired, we considered the possibility that this cytokine may be dispensable for resistance in the absence of PI3K signaling. Therefore, we treated infected p110δ^D910A mice with neutralizing anti-IFN-γ Ab and assessed the outcome of infection over time. As shown in Fig. 5, A and B, treatment with anti-IFN-γ mAb significantly increased lesion size and parasite burden in p110δ^D910A mice, suggesting that as in WT mice, IFN-γ is critically important for resistance of p110δ^D910A mice to L. major.

Many leukocyte types, including B cells and APCs (macrophages and DCs), express the p110δ isoform of PI3K. Because we found that the enhanced resistance of p110δ^D910A mice to L. major was unrelated to differences in B cell and macrophage functions, we determined if defects in T cells are primarily responsible. We used a system where p110δ signaling is intact in leukocytes other than T cells by adoptively transferring purified CD3⁺ T cells (>99% pure) from WT and p110δ^D910A mice into SCID mice that were then infected with L. major. As previously reported (34), SCID recipients of WT T cells became susceptible to L. major as evidenced by the development of large ulcerative lesions and uncontrolled parasite proliferation (Fig. 5, C and D). In contrast, SCID recipients of T cells from p110δ^D910A mice were highly resistant, which associated with lower production of IFN-γ and IL-10 by cells from the spleen and dLNs (Fig. 5,E and data not show). Interestingly, the expansion of CD4⁺CD25⁺Foxp3⁺ cells in SCID mice that received cells from p110δ^D910A mice was severely impaired (Fig. 5 F). Taken together, these results indicate that the defect in p110δ^D910A mice that results in enhanced resistance to L. major is T cell intrinsic. They further suggest that signaling via the p110δ isoform of PI3K in T cells may be important for expansion of Tregs in L. major-infected mice.

p110δ^D910A mice have impaired expansion of Tregs (11, 20). Tregs have been shown to promote parasite persistence and disease chronicity following L. major infection (6, 7, 35). Because we found greatly reduced numbers of CD4⁺CD25⁺Foxp3⁺ cells in SCID mice reconstituted with cells from p110δ^D910A mice, we determined whether the enhanced resistance of p110δ^D910A mice to L. major is related to impaired induction and/or expansion of Tregs. As shown in Fig. 6, A–D, the percentages of CD4⁺CD25⁺ T cells in the spleens, lymph nodes, and infected footpads of p110δ^D910A mice were ∼3- to 4-fold lower than in WT mice. Most of the CD4⁺CD25⁺ T cells in infected mice also expressed the transcription factor Foxp3 (Fig. 6, A and B), a key Treg signature gene (36, 37). Most of the IL-10-producing cells in infected mice were CD3⁺, and the majority of these IL-10 producers were from the CD4⁺CD25⁺ population (Fig. 6,C). Interestingly, the expression of Blimp-1, a T cell lineage-specific transcription factor that plays a role in the function of Tregs (38, 39), was completely absent in activated T cells from p110δ^D910A mice (Fig. 6 D). These results suggest that impaired expansion and/or function of Tregs may be responsible for the enhanced resistance of p110δ^D910A mice to L. major.

Next, we investigated whether the paucity of p110δ^D910A mice Tregs could account for their enhanced resistance to L. major. We injected 2 million CD4⁺CD25⁺ cells (>98% pure) into naive p110δ^D910A mice and infected them with L. major the next day. p110δ^D910A mice that received naive or infected WT CD4⁺CD25⁺ cells lost their enhanced resistance and became susceptible to L. major (Fig. 7, A and B). Interestingly, adoptive transfer of CD4⁺CD25⁻ cells from infected WT mice into p110δ^D910A mice also abolished their enhanced resistance to L. major, and this was associated with increased numbers of CD4⁺CD25⁺ cells in these mice (Fig. 7, C and D, and data not shown).

Next, we investigated whether impaired function of CD25⁺ T cells in p110δ^D910A mice was due to impaired expansion, intrinsic suppressive defects, or both. Adoptive transfer of CD25⁺ cells from infected (but not naive) p110δ^D910A mice into naive p110δ^D910A mice also abolished their enhanced resistance to L. major (Fig. 7, E and F), suggesting that the few CD25⁺ cells generated in infected p110δ^D910A mice are nonetheless functionally suppressive in vivo. In contrast to WT cells, CD4⁺CD25⁻ cells from infected p110δ^D910A mice were unable to transfer susceptibility to naive p110δ^D910A mice (Fig. 7, G and H). Taken together, these results suggest that impaired expansion and recruitment of CD4⁺CD25⁺ cells (i.e., inducible Tregs) in p110δ^D910A mice could largely account for their enhanced resistance to L. major.

The immune response against L. major has served as the single most important paradigm to explore the Th1-Th2 dichotomy. Th1-prone mice are protected, whereas Th2-prone mice are susceptible. Our expectation was therefore that p110δ^D910A mice, which show attenuated Th1 responses, would be susceptible to Leishmania. The opposite is true; that is, they are protected against infection even in the normally susceptible BALB/c background. Our work therefore challenges the current paradigm and instead focuses the attention toward regulatory mechanisms that control inflammation. Moreover, this is the first time the inhibition of a kinase has been shown to offer protection against leishmaniasis (and indeed any parasitic infection). Other than going against the dogma that the quantity and quality of Th1/Th2 cell responses regulate the outcome of infection with L. major and perhaps other intracellular infections, our studies also highlight the importance of the p110δ isoform of PI3K signaling in the regulation of T cell-mediated immunity. These findings have important and direct implications for immunomodulation and immunotherapy in vivo.

A robust IFN-γ response is generally thought to be critical for resistance to L. major (40, 41) by activating macrophages to produce NO, an effector molecule for killing intracellular parasites, and inhibition of IL-4 and IL-10 production by Th2 cells. However, more recent studies suggest that events distinct from the quantity and quality of Th1/Th2 cell responses might play a more dominant role in regulating the outcome of infection with L. major and perhaps other intracellular infections. We previously showed that although BALB/c mice infected with nonpathogenic phosphoglycan-deficient (termed lpg2⁻) L. major do not produce any significant IFN-γ recall responses, and these mice were strongly protected against virulent L. major challenge (42). Similarly, C57BL/6 mice infected with L. major clone SD (MHOM/SN/74/SD) developed chronic nonhealing lesions despite mounting very strong Leishmania-specific IFN-γ responses, and resolution of cutaneous lesions occurred only after blockade of IL-10 or depletion of CD4⁺CD25⁺ Tregs (43). Together with previous studies, our data on p110δ^D910A mice suggest that a strong T cell and IFN-γ response may be required for resistance to L. major only in situations where a substantial number of Tregs are induced. In the absence of optimal Treg activation (as in p110δ^D910A mice), low levels of IFN-γ can more efficiently activate macrophages, leading to more effective intracellular parasite killing in vivo. Indeed, we found that neutralization of IFN-γ by injecting monoclonal anti-IFN-γ Ab into L. major-infected p110δ^D910A mice results in progressive disease associated with uncontrolled parasite proliferation. This indicates that the low levels of IFN-γ produced by the T cells in infected p110δ^D910A mice are nonetheless required for effective parasite control and enhanced resistance to L. major infection. The regulation of excessive IFN-γ production in the absence increased numbers of Tregs makes physiologic sense because this cytokine has been associated with many deleterious side effects and death in many diseases, including toxoplasmosis (44), trypanosomiasis (45, 46, 47), and many viral infections (48, 49).

There is accumulating evidence suggesting that CD4⁺CD25⁺ Tregs play important roles in resistance to many pathogens (6, 35, 50). IL-10-producing natural Tregs accumulate at the primary site of L. major infection in both humans and mice (6, 7, 51), mediate disease chronicity (6, 7, 35), and their depletion leads to parasite clearance (7, 52). We found lower numbers of CD4⁺CD25⁺Foxp3⁺ and Ag-specific IL-10⁺CD4⁺CD25⁺ T cells in the peripheral lymphoid organs and at infection sites of p110δ^D910A mice, indicating that defects in homing, expansion, and/or function of Tregs may contribute to the enhanced resistance of p110δ^D910A mice to L. major. However, because p110δ^D910A mice have impaired peripheral T cell expansion in vivo (21) (see Fig. 2), it is possible that defective expansion of non-Tregs (CD4⁺CD25⁻) may also contribute to the enhanced resistance of p110δ^D910A mice to L. major. Indeed, recent reports have indicated that CD4⁺CD25⁻Foxp3⁻ IL-10-producing cells play important roles in regulating the outcome of parasitic infections, including L. major (8) and Toxoplasma gondii (53). Interestingly, while CD4⁺CD25⁺ cells from infected WT and p110δ^D910A mice were able to transfer susceptibility to naive p110δ^D910A mice, only CD4⁺CD25⁻ cells from infected WT mice were able to transfer susceptibility to p110δ^D910A mice, and this was associated with increased numbers of CD25⁺ cells in the lymph nodes and site of infection. These observations support the conclusion that CD4⁺ T cells from p110δ^D910A mice are intrinsically defective in differentiating into inducible Tregs following infection with L. major.

Several cell types, including macrophages, neutrophils, and DCs, express the p110δ isoform of PI3K and thus could potentially contribute to the observed phenotype of p110δ^D910A mice to L. major infection. Using adoptive transfer studies, we critically confirmed that defects in T cells (specifically impaired expansion of Tregs) are responsible for the enhanced resistance of p110δ^D910A mice to L. major. The enhanced resistance of p110δ^D910A mice could be reproduced in SCID mice by adoptive transfer of highly purified CD3⁺ cells from p110δ^D910A mice. In contrast, transfer of CD4⁺CD25⁺ T cells from naive (uninfected) and infected WT mice into p110δ^D910A mice abolished their enhanced resistance, leading to susceptibility to L. major. Although macrophages and DCs from p110δ^D910A mice produce more IL-12 when stimulated with LPS in vitro, they did not produce more NO (the major effector molecule for killing intracellular Leishmania parasites) or reactive oxygen species than did cells from WT mice. Similarly, macrophages and DCs from p110δ^D910A mice were equally permissive to infection and were not better at killing L. major than were WT cells following stimulation with IFN-γ of LPS in vitro. Cumulatively, these observations implicate intrinsic T cell defects in p110δ^D910A mice as the major contributor of their enhanced resistance to L. major.

Previously, we showed that p110δ^D910A mice have higher numbers of CD4⁺CD25⁺Foxp3⁺ T cells in their thymus than do WT mice (11). In contrast, the numbers of both CD4⁺CD25⁺ and CD4⁺CD25⁻ T cells in the dLNs and spleens of infected p110δ^D910A mice are lower than in those of infected WT mice, indicating that p110δ signaling plays an important role in the peripheral expansion of effector cells and Tregs. Interestingly, we found that while CD25⁺ cells from naive and infected WT mice could abrogate enhanced resistance of p110δ^D910A mice to L. major, only those from infected p110δ^D910A mice were capable of abrogating enhanced resistance of p110δ^D910A mice to L. major. Thus, our data suggest a role for p110δ in peripheral induction and expansion of inducible Tregs. In line with this, we have found that the expression of Blimp-1 is completely absent in activated T cells from p110δ^D910A mice. T cell lineage-specific Blimp-1-deficient mice develop T cell hyperproliferative disorders, including mild colitis (similar to p110δ^D910A mice), due to impaired expansion and function of Tregs (38, 39). Hence, signaling via p110δ may control expansion of Tregs by regulating Blimp expression on T cells. Alternatively, signaling via p110δ may regulate other transcription factors such as Foxo3a and Foxj1, which have been shown to inhibit T cell activation, and their absence results in T cell hyperproliferation and multiorgan inflammation (54, 55). In line with this, p110δ^D910A mice also show a concomitant profound and generalized defective Th cell clonal expansion and differentiation, and this may help to protect them from severe autoimmune manifestations that would otherwise occur.

The finding that p110δ^D910A mice have impaired expansion of T cells in vivo has important therapeutic implications for regulation and/or treatment of diseases caused by excessive or chronic activation of Th1 and Th2 cells. Indeed, pharmacologic blockade of p110δ in mice ameliorates symptoms of allergic airway inflammation and smooth muscle hyperresponsiveness (56), and selective inhibition of PI3K is proposed to yield beneficial effects in thrombosis (57) and other T cell-mediated autoimmune diseases (58, 59). However, it is conceivable that the dampening effects on inflammation seen in L. major-infected p110δ^D910A mice may be specific to this infection model. It is possible that in other inflammatory conditions, inhibition of p110δ signaling may have opposite effects from those reported herein, possibly by altering recruitment and retention of inflammatory cells to inflammatory sites. Current treatment of human cutaneous leishmaniasis involves the use of highly toxic pentavalent antimonial compounds, but treatment failures and drug resistance are common (60). Given the dramatic hyperresistance seen in p110δ^D910A mice infected with L. major, we speculate that pharmacological inhibitors of p110δ may have beneficial effects in the treatment of human cutaneous leishmaniasis, despite therapies currently being developed with antiinflammation as a therapeutic indication (59). Indeed, we have preliminary evidence showing that specific blockade of p110δ signaling with a pharmacologic inhibitor ameliorates disease outcome in L. major-infected mice (data not shown).

In summary, we have demonstrated the critical role of the p110δ isoform of PI3K in controlling the outcome of L. major infection by regulating the expansion of Tregs. Mice with inactive p110δ have fewer CD4⁺CD25⁺Foxp3⁺ T cells and are highly resistant to L. major despite having an impaired in vivo development of both Th1 and Th2 cells. We propose that an impaired Treg activity in p110δ^D910A mice leads to effective and unopposed macrophage function in vivo, resulting in more efficient parasite control and enhanced resistance to L. major.

We thank Drs Peter Bretscher and Henry Tabel for critically reading the manuscript and making useful suggestions.

The authors have no financial conflicts of interest.